I sprayed the ants. They disappeared for two days and came back in a line that looked like a highway.

That is what spraying does. You kill the ones you can see, which are a tiny fraction of the colony, and the rest keep coming.

Follow the trail first

Do not kill anything yet. Watch where the line comes from and where it goes. The entry point is the whole problem.

Ants leave a pheromone trail. Every ant that walks it strengthens it, which is why the line gets thicker instead of thinner.

Clean the trail with vinegar

Mix one part white vinegar to one part water and wipe the entire trail, plus a foot in every direction. This erases the pheromone, and the scouts lose the map.

Then seal the entry with a bead of caulk, petroleum jelly, or even a smear of cinnamon if the gap is tiny.

Bait instead of spray

Bait works because they carry it home. Sugar based bait for the small black ants you see in kitchens, protein based bait for the larger ones.

Place bait stations along the trail, not on it. Leave them alone for a week. You will see more ants for two or three days while they recruit, and then the traffic stops.

Fighting the urge to spray during that window is the hard part. Spraying kills the carriers and resets the whole job.

Cut off the water and food

Ants come inside for moisture as often as food. A dripping faucet or a damp sponge in the sink is enough to keep a colony interested.

Wipe counters dry at night. Rinse cans before recycling. Put pet food bowls in a shallow dish of water, which works as a moat.

Outdoors

Look for the nest. Anthills near the foundation, cracks in the driveway, and the base of shrubs are all common. Boiling water on a nest outside is cheap, brutal, and effective.

Do not destroy every ant outside though. Most of them compete with the ones you actually do not want.